1. Field of the Invention
This invention relates to an automatic transmission control system for a vehicle.
2. Description of the Prior Art
In an automatic transmission, a torque convertor and a transmission mechanism are connected and the gear is shifted by selectively engaging and disengaging a plurality of friction coupling elements to change the power transmission path through the transmission mechanism. There has been known an automatic transmission in which the transmission mechanism comprises a main transmission mechanism and a sub transmission mechanism so that the speed of the output of the main transmission mechanism is further changed by the sub transmission mechanism in order to obtain an increased number of gear speeds.
In such an automatic transmission, the main transmission mechanism and the sub transmission mechanism sometimes shift simultaneously with each other. In such a case, when their shifting actions do not synchronize with each other, an uncomfortable torque shock occurs. When their shifting actions are reverse to each other, i.e., one upshifts and the other downshifts, the torque shock is more significant. In order to overcome the problem, it has been proposed, for instance, in Japanese Unexamined Patent Publication No. 61(1986)-99745 to cause the sub transmission mechanism to initiate and complete a downshift during an upshift of the main transmission mechanism.
However, the torque generated when the main and sub transmission mechanisms shift in opposite directions cannot be sufficiently suppressed by simply causing one of the main and sub transmission mechanisms to initiate and complete a shifting action during that of the other, but it is necessary to harmonize the change in the gear ratio in one of the transmission mechanisms with that in the other transmission mechanism in order to efficiently suppress the torque shock.
This may be realized by employing an electrically controlled hydraulic control means such as a linear solenoid valve or a duty solenoid valve to control engagement and disengagement of a friction coupling element in one transmission mechanism, detecting the gear ratio of the transmission mechanism and feedback-controlling the engagement and disengagement of the friction coupling element so that the gear ratio of the transmission mechanism changes in harmony with change in the gear ratio of the other transmission mechanism.
However, use of such an electrically controlled hydraulic control means gives rise to the following problem. That is, when such a hydraulic control means is used, the control hydraulic pressure is apt to vary or fluctuate. Further since it is generally designed to handle a relatively small amount of working oil, its controlling accuracy deteriorates when the amount of the working oil is large. Accordingly, such an electrically controlled hydraulic control means, if may be useful during the gear-shifting which requires the feedback control described above, can adversely affect the other gear-shiftings in which the friction coupling element is involved.